I found my favourite drive, except for the Jump Drive, to be the modified Warp Drive. Now I wish to know:

The Warp Drive uses 1 fuel uniot per jump, the Scram Drive uses 1.5 units per jump, the Jump Drive uses 2 units per jump but the MWD uses 0.75 units per jump. Does it make a difference in fuel consumption, to install a MWD and SD (or JD) together; and if so, for better or for worse?

I need to ask in the forums, because I'm currently permanently indebted to my crews and all income go directly into arrears salaries.

It sounds like you're using a plugin. It's difficult to answer your question for certain without seeing hte definition of the "Modified Warp Drive".
But, for vanilla outfits, having a hyperdrive and scram drive installed will always use the scram drive fuel cost when jumping along a hyperlane. Having a jump drive installed alongside a hyperdrive of scram drive will use the jump drive fuel cost when jumping between unliked systems. So, there is a benefit to having a jump drive and either a hyperdrive and scram drive installed because hte latter will allow you to jump along hyperlinks for a lower fuel cost.